Support @Contract annotations

[wmdietl]

To provide better interaction with IntelliJ, NullAway, etc, it would be nice to support @Contract annotations and interpret them as the corresponding CF annotations.
For documentation see:

https://github.com/JetBrains/java-annotations/blob/master/src/jvmMain/java/org/jetbrains/annotations/Contract.java

We do not want to add an extra dependency on this annotation, so we need to find a solution, e.g. having our own version and creating an alias.
We also need to be careful about the possible performance impact.